This invention relates to a device for inspecting a surface of a disc-shaped recording medium, such as an optical disc or a magneto-optical disc, and, above all, that of a mold used for producing the disc-shaped recording medium, such as a metal master or a stamper, in which the recording medium or the mold has lands and grooves. The device is used in an apparatus for producing the disc-shaped recording medium or the mold.
The disc-shaped recording medium, such as a magnetic disc, which is recorded or reproduced using a slide type recording/reproducing head, is required to have a smooth sliding surface in consideration of performance and durability of the recording/reproducing head. Such a disc-shaped recording medium is in need of surface inspection in the course of a manufacturing process thereof for detecting the possible presence of protuberances on the sliding surface on which slides the recording/reproducing head for providing a basis for judgement as to acceptability thereof as to the above-mentioned surface smoothness.
In surface inspection of the disc-shaped recording medium, the conventional practice has been to check the entire slide surface thereof on which slides the recording/reproducing head with the aid of an optical microscope or a picture processing device for detecting the presence of foreign matter. Although a high inspection capability can be achieved with these methods as to the presence of foreign matter, the information along the height of the foreign matter cannot thereby be obtained.
In the field of the optical disc, inspection with the aid of a laser beam is carried out in addition to visual inspection with the aid of an optical microscope. That is, a laser beam from a laser light source on the order of tens to hundreds of micrometers is converged by a laser light optical system on an optical disc surface and spirally swept thereon for inspecting the recording surface of the optical disc. With this method, since the variation in the light volume is averaged depending on the beam spot size, delicate variations in the minute-sized lands or grooves on the reflecting surface cannot be detected.
There has also been known a surface inspection device for inspecting the surface of the disc-shaped recording medium in which a laser spot, converged to a beam size on the order of several micrometers using the laser optical system, is simply swept in a spiral path for achieving the inspection of the entire surface of the recording medium.
If the surface inspection device for the optical disc-shaped recording medium employing the laser beam optical system is applied to a recording medium having grooves formed thereon, the light volume of the reflected light from the disc surface is changed significantly at a time instant the laser spot traverses the groove and the land respectively representing the recess and the protuberance formed on the disc surface, unless the tracking control is in operation. There may be occasions wherein the variation in the light volume from the lands and grooves on the disc surface becomes invisible. The surface inspection device for the disc-shaped recording medium having such grooves as recesses is in need of tracking control for surface inspection, that is, the laser spot has to be controlled so as to follow the land sandwiched between two grooves as the laser spot is caused to sweep the entire disc surface.
As shown in JP Laid Open Patent Publication No. JP-A-61-148636, provisionally published on Jul. 7, 1986, such tracking control is a technique commonly adopted with an optical disc. Such tracking control is now explained by referring to a tracking control circuit applied to a stamper inspection device, as shown in FIG. 1.
In general, a light receiving unit 200 for detecting the light reflected by an optical disc surface has six-segment light receiving elements A to F for receiving the reflected light of a main beam (O'th order light) S.sub.M and two sub-beams (.+-.1st order light). These two sub-beams are made up of a preceding sub-beam (+1st order light) preceding the main beam in its proceeding direction and a follow-up sub-beam (-1st order light) S.sub.-1 positioned following the main beam. These two sub-beams S.sub.+1 and S.sub.-1 are radiated so as to be located on a track inclined by a angle .+-..theta. with respect to a track TR.sub.m, as shown for example in FIG. 2.
For tracking control, the light receiving unit 200 employs detection outputs of the light receiving elements E and F, among the six-segment light receiving elements, which are configured to detect the reflected light from the leading sub-beam and the trailing sub-beam. These detection outputs are fed to a current/voltage converter 201. The current/voltage converter 201 includes current/voltage converting circuits 201e, 201f for conversion. The resulting converted signals are fed to a tracking error signal detection circuit 202.
The tracking error signal detection circuit 202 has an adder 202a and a phase compensation circuit 202b. An output signal of the light receiving element E is additively entered to an input of the addition unit 202a, while an output signal of the light receiving element F is subtractively entered to its other input. A tracking error signal is derived from these input signal inputs. The phase compensation circuit 202b performs phase compensation on the tracking error signal and transmits the phase-compensated signal to a fixed terminal a of a mode changeover switch 203. The phase compensation circuit 202b transmits the tracking error signal to a zero-crossing detection unit 204.
The zero-crossing detection unit 204 has an inverting circuit 204a, two differentiation circuits 204b, 204c and two diodes 204d, 204e. The zero-crossing detection unit 204 routes the tracking error signal from the phase compensation circuit 202b to the inverting circuit 204a and to the differentiating circuit 204c. The inverting circuit 204a inverts the signal level of the tracking error signal and transmits the inverted signal to the differentiating circuit 204b. The differentiating circuits 204d, 204c take out only positive terminal side signals as signals indicating the maximum and minimum positions of the supplied signals and transmit the signals to a jump control circuit 205.
The jump control circuit 205 issues an acceleration pulse for track jumping a laser beam spot radiated from e.g., an optical pickup unit, and subsequently transmits a deceleration pulse to the other fixed terminal b of the mode changeover switch 203. The jump control unit 205 also effects timing control in supplying these various pulse signals.
The mode changeover switch 203 is changed over responsive to a mode changeover signal changing over the tracking control mode to the track jump mode and vice versa. The mode changeover switch 203 issues the signals supplied to its fixed terminals as a tracking control signal.
If, with the above constitution, the tracking error signal is found based upon output signals from various parts, an output signal of the light receiving element E responsive to the leading sub-beam S.sup.+1 as shown in FIG. 3a and an output signal of the light receiving element F responsive to the trailing sub-beam S.sup.-1 as shown in FIG. 3c are outputted as signals 180 out of phase relative to each other. The output signals of the light receiving elements E and F are respectively entered at the adder 202a additively and subtractively for producing a tracking error signal as shown in FIG. 3d.
This tracking error signal is routed to the zero-crossing detection unit 204. The differentiating circuits 204b, 204c of the zero-crossing detection unit 204 differentiate the signal level of the tracking error signal for detecting the peak of the tracking error signal. The peak position of the tracking error signal represents the zero-crossing position of a high frequency (HF) signal obtained on summing the output signals of the four-segment light receiving elements A, B, C and D, as shown in FIG. 3.
In addition, the zero-crossing detection unit 204 supplies a positive polarity pulse signal to the jump control unit 205 as a zero-crossing signal via diodes 204d, 204e. In the jump control circuit 205, the accelerating pulse impression start timing and the decelerating pulse impression start timing for jumping over plural tracks can be accurately produced based upon the supplied pulse signal.
Among disc-shaped recording media, there is such a recording medium in which a convexed portion termed a land has a radial width larger than the laser spot size and a concave portion on the lateral side of the land termed a groove has a radial width substantially equal to the laser spot size, as shown in FIG. 4.
If, with such disc-shaped recording medium, the laser spot traverses a track, the light volume of the reflected light from the disc surface having such lands and grooves is not a repetition of increased light volume and decreased light volume with the reflected light volume from the lands being maximum and the light volume reflected from the grooves being minimum. In effect, the reflected light volume becomes minimum at the land-groove boundary region, and becomes only slightly smaller on the groove than on the land.
The tracking error signal produced on receiving the reflected light from the disc-shaped recording medium is obtained as an irregular waveform signal having a period twice that of the prior-art tracking error signal by setting the land width so as to be unequal to the groove width, such as by setting the land width to groove width ratio to 2:1. If such tracking error signal is employed, it is not possible with the conventional tracking control technique to cause the laser spot to follow the track with sufficiently accuracy.